Introduction

At Yelp we rely heavily on pre-commit hooks to find and fix common issues before
changes are submitted for code review. We run our hooks before every commit to
automatically point out issues like missing semicolons, whitespace problems, and
testing statements in code. Automatically fixing these issues before posting
code reviews allows our code reviewer to pay attention to the architecture of a
change and not worry about trivial errors.

As we created more libraries and projects we recognized that sharing our pre
commit hooks across projects is painful. We copied and pasted bash scripts
from project to project and had to manually change the hooks to work for
different project structures.

We believe that you should always use the best industry standard linters. Some
of the best linters are written in languages that you do not use in your project
or have installed on your machine. For example scss-lint is a linter for SCSS
written in Ruby. If you’re writing a project in node you should be able to use
scss-lint as a pre-commit hook without adding a Gemfile to your project or
understanding how to get scss-lint installed.

We built pre-commit to solve our hook issues. It is a multi-language package
manager for pre-commit hooks. You specify a list of hooks you want and
pre-commit manages the installation and execution of any hook written in any
language before every commit. pre-commit is specifically designed to not require
root access. If one of your developers doesn’t have node installed but modifies
a JavaScript file, pre-commit automatically handles downloading and building node
to run jshint without root.

Installation

Before you can run hooks, you need to have the pre-commit package manager installed.

Using pip:

pip install pre-commit

Non Administrative Installation:

curl http://pre-commit.com/install-local.py | python

System Level Install:

curl https://bootstrap.pypa.io/get-pip.py | sudo python - pre-commit

In a Python Project, add the following to your requirements.txt (or requirements-dev.txt):

pre-commit

Adding pre-commit plugins to your project

Once you have pre-commit installed, adding pre-commit plugins to your project is done with the .pre-commit-config.yaml configuration file.

Add a file called .pre-commit-config.yaml to the root of your project. The pre-commit config file describes:

This configuration says to download the pre-commit-hooks project and run its trailing-whitespace hook.

Usage

Run pre-commit install to install pre-commit into your git hooks. pre-commit will now run on every commit. Every time you clone a project using pre-commit running pre-commit install should always be the first thing you do.

If you want to manually run all pre-commit hooks on a repository, run pre-commit run --all-files. To run individual hooks use pre-commit run <hook_id>.

The first time pre-commit runs on a file it will automatically download, install, and run the hook. Note that running a hook for the first time may be slow. For example: If the machine does not have node installed, pre-commit will download and build a copy of node.

Creating new hooks

pre-commit currently supports hooks written in JavaScript (node), Python, Ruby and system installed scripts. As long as your git repo is an installable package (gem, npm, pypi, etc.) or exposes an executable, it can be used with pre-commit. Each git repo can support as many languages/hooks as you want.

An executable must satisfy the following things:

Returncode of hook must be different between success / failures (Usually 0 for success, nonzero for failure)

Supported languages

Advanced features

Running in migration mode

By default, if you have existing hooks pre-commit install will install in a migration mode which runs both your existing hooks and hooks for pre-commit. To disable this behavior, simply pass -f / --overwrite to the install command. If you decide not to use pre-commit, pre-commit uninstall will restore your hooks to the state prior to installation.

Temporarily disabling hooks

Not all hooks are perfect so sometimes you may need to skip execution of one or more hooks. pre-commit solves this by querying a SKIP environment variable. The SKIP environment variable is a comma separated list of hook ids. This allows you to skip a single hook instead of --no-verifying the entire commit.

$ SKIP=flake8 git commit -m "foo"

pre-commit during commits

Running hooks on unstaged changes can lead to both false-positives and false-negatives during committing. pre-commit only runs on the staged contents of files by temporarily saving the contents of your files at commit time and stashing the unstaged changes while running hooks.

pre-commit during merges

The biggest gripe we’ve had in the past with pre-commit hooks was during merge conflict resolution. When working on very large projects a merge often results in hundreds of committed files. I shouldn’t need to run hooks on all of these files that I didn’t even touch! This often led to running commit with --no-verify and allowed introduction of real bugs that hooks could have caught. pre-commit solves this by only running hooks on files that conflict or were manually edited during conflict resolution.

pre-commit during push

As of version 0.3.5, pre-commit can be used to manage pre-push hooks. Simply pre-commit install --hook-type pre-push.

Passing arguments to hooks

Sometimes hooks require arguments to run correctly. You can pass static arguments by specifying the args property in your .pre-commit-config.yaml as follows:

If the args property is empty or not defined, you should expect your script to be called:

path/to/script-or-system-call dir/file1 dir/file2 file3

Overriding Language Version

Sometimes you only want to run the hooks on a specific version of the language. For each language, they default to using the system installed language (So for example if I’m running python2.6 and a hook specifies python, pre-commit will run the hook using python2.6). Sometimes you don’t want the default system installed version so you can override this on a per-hook basis by setting the language_version.

Usage in Continuous Integration

pre-commit can also be used as a tool for continuous
integration. For instance, adding
pre-commit run --all-files as a CI step will
ensure everything stays in tip-top shape.

Contributing

We’re looking to grow the project and get more contributors especially to support more languages/versions. We’d also like to get the hooks.yaml files added to popular linters without maintaining forks / mirrors.